Method for starting an engine

ABSTRACT

A method for improving starting of an engine that may be repeatedly stopped and started is presented. In one embodiment, the method skips combustion in at least one cylinder, according to the engine combustion order, to control engine speed when an engine is automatically restarted. The skipped combustion event may be related to a level of IMEP when combustion in a cylinder occurs under an operating condition.

The present application is a continuation of U.S. patent applicationSer. No. 12/707,567 filed Feb. 17, 2010, now U.S. Pat. No. 7,931,002,the entire contents of which are incorporated herein by reference.

FIELD

The present description relates to a system for improving starting of anengine. The method may be particularly useful for engines that are oftenstopped and then restarted.

BACKGROUND AND SUMMARY

Vehicle fuel economy may be improved by selectively stopping andstarting the engine of a vehicle. The engine may be stopped while thevehicle is in heavy stop-and-go traffic or at stop lights, for example.Recently such engine operation has been proposed for engines coupled toautomatic transmissions. However, stopping and restarting an engine maybe challenging for engines coupled to an automatic transmission becauseof characteristics of a torque converter that may be placed between theengine crankshaft output and the transmission input. Specifically,torque converter output torque increases more rapidly as engine speedexceeds a threshold speed. If an engine is restarted and allowed toexceed the threshold speed, an increasing amount of engine torque can betransferred to the vehicle driveline and wheels. As a result, it may bepossible to impart more torque from the engine to the vehicle wheelsduring an engine restart than is desired.

At some engine operating conditions torque converter output can becontrolled by adjusting engine speed. Engine speed may be controlled byretarding and/or advancing spark delivered to engine cylinders. Further,under some engine operating conditions it may be possible to controlengine speed by controlling the engine air-fuel mixture from whichengine torque is generated. However, there may be engine operatingconditions when engine cylinders are restricted to a threshold indicatedmean effective pressure (IMEP) to consistently operate engine cylinders.For example, some fuel injectors require a minimum pulse width in orderto inject an expected amount of fuel to a cylinder of the engine. If theinjector is operated at a smaller pulse width, the engine cylinder maynot receive fuel or the amount of fuel received may not be sufficient tosupport combustion in the cylinder. On the other hand, if the fuelinjector is operated at the minimum pulse width, cylinder pressure mayexceed a desired IMEP value. As a result, the engine may accelerateabove the engine speed where an increased amount of engine torque may betransferred through the torque converter and to the vehicle wheelsduring an engine start while the transmission is in gear. Consequently,it may be difficult under some conditions to control engine speed duringan engine start so that engine speed does not exceed a threshold leveland cause the torque converter to transfer an undesirable amount ofengine torque to the vehicle wheels.

The inventors herein have recognized the above-mentioned disadvantagesand have developed a method for improving engine starting. Oneembodiment of the present description includes a method for starting anengine, comprising: stopping the engine; automatically initiating anengine restart and initiating combustion in a first cylinder of theengine; and skipping combustion, according to an order of combustion ofthe engine, in at least one cylinder of the engine during the enginerestart after initiating combustion in the first cylinder.

By skipping a combustion event during the restart of an engine that iscoupled to an automatic transmission and started in gear, it may bepossible to control engine speed so that an undesirable amount of enginetorque is not transferred to the wheels of a vehicle. For example,during an engine restart, combustion may be initiated in a cylinder.Combustion may proceed in other engine cylinders according to the enginecombustion order (e.g., 1-3-4-2 for a four-stroke four cycle engine).However, combustion in one or more cylinders according to the combustionorder may be inhibited so that engine speed approaches a desired level.In one example, combustion may be initiated in a particular cylinder ofan engine. The next cylinder in the combustion order may also combust anair-fuel mixture while the third cylinder according to the enginecombustion order proceeds through a cylinder cycle without combustion anair-fuel mixture. In this way, it may be possible to control enginespeed and the transfer of torque from an engine to vehicle wheels duringa start of an engine coupled to an automatic transmission that is ingear.

The present description may provide several advantages. In particular,the approach may reduce the possibility of transferring an undesirableamount of engine torque to vehicle wheels during an engine start.Further, the approach may improve engine speed control during enginestarting irrespective of the type of transmission coupled to the engine.Further still, the approach may be able to compensate for enginehardware that may require a cylinder IMEP that is higher than isdesirable during an engine start.

The above advantages and other advantages, and features of the presentdescription will be readily apparent from the following DetailedDescription when taken alone or in connection with the accompanyingdrawings.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages described herein will be more fully understood by readingan example of an embodiment, referred to herein as the DetailedDescription, when taken alone or with reference to the drawings,wherein:

FIG. 1 is a schematic diagram of an engine;

FIG. 2 is an example plot of a simulated engine start sequence;

FIG. 3 is an example plot of an alternative engine start sequence;

FIG. 4 is an example of a simulated alternative engine start sequence;

FIG. 5 is a flow chart of an engine starting routine; and

FIG. 6 is a flow chart of an alternative engine starting routine.

DETAILED DESCRIPTION

Automatically restarting of an engine may be particularly challengingfor engines equipped with automatic transmissions. The engine of FIG. 1may be started by the methods of FIGS. 5 and 6 as illustrated bystarting sequences of FIGS. 2-4 to improve starting of an engine coupledto an automatic transmission. The systems and methods described hereinmay offer improved engine speed control during starting so that anengine may be restarted when a transmission coupled to the engine isengaged in a drive gear without sending an undesirable amount of enginetorque to the vehicle wheels.

Referring to FIG. 1, internal combustion engine 10, comprising aplurality of cylinders, one cylinder of which is shown in FIG. 1, iscontrolled by electronic engine controller 12. Engine 10 includescombustion chamber 30 and cylinder walls 32 with piston 36 positionedtherein and connected to crankshaft 40. Combustion chamber 30 is showncommunicating with intake manifold 44 and exhaust manifold 48 viarespective intake valve 52 and exhaust valve 54. Each intake and exhaustvalve may be operated by an intake cam 51 and an exhaust cam 53.Alternatively, one or more of the intake and exhaust valves may beoperated by an electromechanically controlled valve coil and armatureassembly. The position of intake cam 51 may be determined by intake camsensor 55. The position of exhaust cam 53 may be determined by exhaustcam sensor 57.

Intake manifold 44 is also shown coupled to the engine cylinder havingfuel injector 66 coupled thereto for delivering liquid fuel inproportion to the pulse width of signal FPW from controller 12. Fuel isdelivered to fuel injector 66 by a fuel system (not shown) including afuel tank, fuel pump, and fuel rail (not shown). The engine 10 of FIG. 1is configured such that the fuel is injected directly into the enginecylinder, which is known to those skilled in the art as directinjection. Fuel injector 66 is supplied operating current from driver 68which responds to controller 12. In addition, intake manifold 44 isshown communicating with optional electronic throttle 62 which controlsthe position of throttle plate 64. Air may enter intake manifold 44 fromair inlet 42 by way of throttle plate 64. In one example, a low pressuredirect injection system may be used, where fuel pressure can be raisedto approximately 20-30 bar. Alternatively, a high pressure, dual stage,fuel system may be used to generate higher fuel pressures.

Distributorless ignition system 88 provides an ignition spark tocombustion chamber 30 via spark plug 92 in response to controller 12.Universal Exhaust Gas Oxygen (UEGO) sensor 126 is shown coupled toexhaust manifold 48 upstream of catalytic converter 70. Alternatively, atwo-state exhaust gas oxygen sensor may be substituted for UEGO sensor126.

Converter 70 can include multiple catalyst bricks, in one example. Inanother example, multiple emission control devices, each with multiplebricks, can be used. Converter 70 can be a three-way type catalyst inone example.

Engine 10 may be coupled to an automatic or manual transmission (notshown) to deliver engine torque to vehicle wheels. In an alternativeembodiment, engine 10 may be part of a hybrid driveline.

Controller 12 is shown in FIG. 1 as a conventional microcomputerincluding: microprocessor unit 102, input/output ports 104, read-onlymemory 106, random access memory 108, keep alive memory 110, and aconventional data bus. Controller 12 is shown receiving various signalsfrom sensors coupled to engine 10, in addition to those signalspreviously discussed, including: engine coolant temperature (ECT) fromtemperature sensor 112 coupled to cooling sleeve 114; a position sensor134 coupled to an accelerator pedal 130 for sensing force applied byfoot 132; a measurement of engine manifold pressure (MAP) from pressuresensor 122 coupled to intake manifold 44; an engine position sensor froma Hall effect sensor 118 sensing crankshaft 40 position; a measurementof air mass entering the engine from sensor 120; and a measurement ofthrottle position from sensor 58. Fuel rail pressure and barometricpressure may also be sensed (sensor not shown) for processing bycontroller 12. In a preferred aspect of the present description, engineposition sensor 118 produces a predetermined number of equally spacedpulses every revolution of the crankshaft from which engine speed (RPM)can be determined.

In some embodiments, the engine may be coupled to an electricmotor/battery system in a hybrid vehicle. The hybrid vehicle may have aparallel configuration, series configuration, or variation orcombinations thereof. Further, engine crankshaft 40 may be rotated by astarter or by a motor of a hybrid vehicle to assist engine starting.

During operation, each cylinder within engine 10 typically undergoes afour stroke cycle: the cycle includes the intake stroke, compressionstroke, expansion stroke, and exhaust stroke. During the intake stroke,generally, the exhaust valve 54 closes and intake valve 52 opens. Air isintroduced into combustion chamber 30 via intake manifold 44, and piston36 moves to the bottom of the cylinder so as to increase the volumewithin combustion chamber 30. The position at which piston 36 is nearthe bottom of the cylinder and at the end of its stroke (e.g. whencombustion chamber 30 is at its largest volume) is typically referred toby those of skill in the art as bottom dead center (BDC). During thecompression stroke, intake valve 52 and exhaust valve 54 are closed.Piston 36 moves toward the cylinder head so as to compress the airwithin combustion chamber 30. The point at which piston 36 is at the endof its stroke and closest to the cylinder head (e.g. when combustionchamber 30 is at its smallest volume) is typically referred to by thoseof skill in the art as top dead center (TDC). In a process hereinafterreferred to as injection, fuel is introduced into the combustionchamber. In a process hereinafter referred to as ignition, the injectedfuel is ignited by known ignition means such as spark plug 92, resultingin combustion. During the expansion stroke, the expanding gases pushpiston 36 back to BDC. Crankshaft 40 converts piston movement into arotational torque of the rotary shaft. Finally, during the exhauststroke, the exhaust valve 54 opens to release the combusted air-fuelmixture to exhaust manifold 48 and the piston returns to TDC. Note thatthe above is shown merely as an example, and that intake and exhaustvalve opening and/or closing timings may vary, such as to providepositive or negative valve overlap, late intake valve closing, orvarious other examples.

In one embodiment, the stop/start crank position sensor has both zerospeed and bi-directional capability. In some applications abi-directional Hall sensor may be used, in others the magnets may bemounted to the target. Magnets may be placed on the target and the“missing tooth gap” can potentially be eliminated if the sensor iscapable of detecting a change in signal amplitude (e.g., use a strongeror weaker magnet to locate a specific position on the wheel). Further,using a bi-directional Hall sensor or equivalent, the engine positionmay be maintained through shut-down, but during re-start alternativestrategy may be used to assure that the engine is rotating in a forwarddirection.

Referring to FIG. 2, an example plot of a simulated engine startsequence by the method of FIG. 5 is shown. In particular, events ofinterest for starting a four-stroke four cylinder engine are shown.Vertical marker 200 represents a reference to time T_(O). At T_(O), andto the left of T_(O), the engine is not rotating and may be referencedby time. Right of T_(O), the engine is rotating as time increases to theright. Engine events (e.g., fuel injection timing and spark) areillustrated with respect to engine position as the engine rotatesthrough an engine cycle. As the engine accelerates the time for anengine stroke decreases, but the engine stoke remains constant in termsof crankshaft angular degrees. FIGS. 2-4 illustrate engine events interms of engine strokes; therefore, the time scale of engine events maychange but the crankshaft angular distance remains constant as shown inFIGS. 2-4.

The engine position of each cylinder of a four cylinder engine isdescribed by the traces labeled CYL. 1-4. The vertical markers along thelength of traces CYL. 1-4 represent top-dead-center andbottom-dead-center piston positions for the respective cylinders. Therespective cylinder strokes of each cylinder are indicated by INTAKE,COMP., EXPAN., and EXH. identifiers.

At T_(O), the engine is stopped on the intake stroke of cylinder number1, the exhaust stroke of cylinder number 3, the expansion stroke ofcylinder number 4, and the compression stroke of cylinder number 2. Inthis example, fuel is injected to cylinder number 2 while the engine isnot rotating during a compression stroke. Injecting before enginerotation and during the compression stroke increases the possibilitythat the engine will start earlier. In this example, the first fuelinjection is identified by the injection window indicated at marker 202.The injection window illustrated is illustrated by a box and is anamount of time necessary to inject the desired amount of fuel into thecylinder, although the injection timings illustrated are exemplary innature only and are not indicative of any particular amount of fuel tobe injected. As can be seen from FIG. 2, the first amount of fuelinjected into cylinder number 2 is injected in a single injection.However, two or more separate injections of fuel into the cylinder tofirst receive fuel since engine stop is possible. Further, in someembodiments, fuel may be injected to the first cylinder to receive fuelafter engine stop after the engine begins to rotate.

A cylinder counter is also started at T_(O). The cylinder counter countsthe number of cylinder events from engine stop. The cylinder counterbegins counting at the cylinder to first receive fuel and continuesincrementing with each cylinder that rotates through bottom-dead-centerintake stroke. For example, the cylinder counter increments to a valueof one at time of first injection at marker 206. The cylinder counter isincremented again, this time to a value of two at 212, and so on as theengine continues to rotate. In alternative embodiments the cylindercounter may be incremented at different engine positions or by differentevents. For example, the cylinder counter may be incremented at tendegrees after the beginning of each cylinder intake stroke.

As the engine rotates, combustion in cylinder number 2 is initiated by aspark at 204; however, in some embodiments, a spark may be initiatedbefore engine rotation, thereby inducing engine rotation before or whilethe starter is engaged. While cylinder number 2 is on a compressionstroke, cylinder number 1 is on an intake stroke. Fuel may be injectedduring the intake stroke of cylinder number 1 and/or during thecompression stroke as illustrated by the dashed lines of the injectionwindow 206. Combustion is initiated in cylinder number 1 at 210 asindicated by the spark 210.

At intake stroke 214 and compression stroke 216, no fuel is injected noris spark provided to the next cylinder in the combustion order, cylindernumber 3. No fuel is injected after the cylinder counter reaches a valueof two until the counter reaches a value of three at which time fuelinjection and spark resume at 218 and 220 respectively. Fuel is againinjected at 222 and 224 while the cylinder counter value is four.Injection and spark are stopped again during intake stroke 226 and 228when the cylinder counter value is five. An engine operating in thismanner may be referred to as operating in skip mode combustion.

Thus, in the example of FIG. 2, the cylinder counter counts the numberof cylinders having rotated through and intake stroke in the order ofcombustion (e.g. 1-3-4-2), the engine controller injects fuel andinitiates spark for two cylinders in order of combustion, the enginecontroller skips injecting fuel and initiating spark for one cylinder inthe order or combustion, the engine controller resumes fuel injectionand initiation of spark for two cylinders in the order of combustion,and then the engine controller skips injecting fuel and initiating sparkfor another cylinder in the order of combustion. However, it should benoted that the order and sequence illustrated by FIG. 2 is onlyexemplary in nature and not intended to limit the scope of thedescription. For example, in some embodiments three cylinders maycombust an air-fuel mixture before combustion is skipped in a cylinder.In other embodiments, four cylinders may combust an air-fuel mixturebefore combustion is skipped in a cylinder. In other embodiments,combustion may be skipped in two cylinders in a row rather than one asdepicted by FIG. 2.

After skipped injection and spark at 226 and 228, FIG. 2 shows fuelinjection is resumed on a continuous basis. Although the depicted methodis effective to control engine speed so that the possibility ofovershooting idle speed is reduced, the present method may be used tocontrol engine idle speed for a longer duration, if desired. Forexample, skip mode combustion may be executed for an predeterminednumber of cylinder cycles or engine cycles after an engine stop beforeall cylinders are continuously combusting air-fuel mixtures.

Turning now to FIG. 3, an example plot of an alternative engine startsequence by the method of FIG. 5 is shown. Similar to FIG. 2, events ofinterest for starting a four-stroke four cylinder engine are shown.Vertical marker 300 represents a reference to time T_(O). At T_(O), andto the left of T_(O), the engine is not rotating and may be referencedby time. Right of T_(O), the engine is rotating as time increases to theright.

The engine position of each cylinder of a four cylinder engine isdescribed by the traces labeled CYL. 1-4. The vertical markers along thelength of traces CYL. 1-4 represent top-dead-center andbottom-dead-center piston positions for the respective cylinders. Therespective cylinder strokes of each cylinder are indicated by INTAKE,COMP., EXPAN., and EXH. identifiers.

The engine position at engine stop, time left of T_(O), is the same asillustrated by FIG. 2. However, during this example engine start, fuelis not injected while the engine is not rotating. The first fuelinjection occurs after the engine begins to rotate to the right of T_(O)at 302. The first spark event is at 304. Similar to FIG. 2, theinjection window is illustrated by a box and is an amount of timenecessary to inject the desired amount of fuel into the cylinder.

A cylinder counter similar to the one described in FIG. 2 is started atT_(O). The cylinder counter counts the number of cylinder events fromengine stop. The cylinder counter begins counting at the cylinder tofirst receive fuel after an engine stop and continues incrementing witheach cylinder that rotates through a particular position,bottom-dead-center intake stroke for example.

The fuel injection amounts of the first three fuel injections 302, 306,and 310 occur late in the compression stroke and are of a shorterduration than the fuel injection of the fourth cylinder to receive fuelat 314. The injection timing is late in the compression stroke andshorter in duration in order to facilitate lean stratified combustionfor the first three cylinder combustion events. Injecting late in thecompression stroke allows a richer mixture to develop around the sparkplug just before spark is initiated so that the air-fuel mixture igniteseven though the amount of fuel injected into the cylinder may notsupport combustion if the air-fuel mixture in the cylinder washomogenous. By combusting a stratified mixture for the first fewcombustion events, it may be possible to generate less engine torque,thereby reducing engine torque so that engine speed does not exceed adesired level during and engine start and run-up (e.g., the portion ofan engine start when the engine is accelerating from crank speed untilidle speed is reached).

In one embodiment, the number of stratified lean combustion events maybe predetermined and stored in memory of an engine controller. Thus, forthe example of FIG. 3, stratified lean combustion is programmed forthree combustion events. The fourth combustion event at 316, is ahomogenous combustion event based on the intake stroke fuel injection at314. Thus, it may be possible to control engine speed during a start byperforming stratified lean combustion and transitioning to homogenouscombustion after a predetermined number of cylinder or combustionevents. After fuel injection at 314, all engine cylinders continue tocombust homogenous mixtures; however, it is possible to operate afraction of engine cylinders in a stratified lean combustion mode ifdesired.

Referring now to FIG. 4, an example plot of an alternative engine startsequence by the method of FIG. 6 is shown. Similar to FIGS. 2 and 3,events of interest for starting a four-stroke four cylinder engine areshown. Vertical marker 400 represents a reference to time T_(O). AtT_(O), and to the left of T_(O), the engine is not rotating and may bereferenced by time. Right of T_(O), the engine is rotating as timeincreases to the right.

The engine position of each cylinder of a four cylinder engine isdescribed by the traces labeled CYL. 1-4. The vertical markers along thelength of traces CYL. 1-4 represent top-dead-center andbottom-dead-center piston positions for the respective cylinders. Therespective cylinder strokes of each cylinder are indicated by INTAKE,COMP., EXPAN., and EXH. identifiers.

The engine position at engine stop, time left of T_(O), is the same asillustrated by FIGS. 2-3. The first fuel injection is shown as partiallyinjected at engine stop and continues as the engine begins to rotate tothe right of T_(O) at 402. However, the first injection event may occurat engine stop or after the engine begins to rotate. The first sparkevent occurs at 406. In this example, there are two fuel injectionevents 402 and 404 during the first cycle of the first cylinder sinceengine stop to receive fuel injection. Similar to FIGS. 2 and 3, theinjection windows are illustrated by boxes and are an amount of timenecessary to inject the desired amount of fuel into the cylinder. Thefirst two injection events occur during the compression stroke ofcylinder number 2. Injecting twice during the compression stroke mayreduce engine starting time and may improve combustion stability for thefirst cylinder to fire (e.g., combust an air-fuel mixture).

A cylinder counter similar to the ones described in FIGS. 2 and 3 isstarted at T_(O). The cylinder counter counts the number of cylinderevents from engine stop. The cylinder counter begins counting at thecylinder to first receive fuel since engine stop and continuesincrementing with each cylinder that rotates through a particularposition, bottom-dead-center intake stroke for example.

After fuel is injected twice for the first cylinder since engine stopand during the compression stroke at 402 and 404, fuel is injected twiceeach cylinder cycle to the remaining cylinders in the engine combustionorder at 408-430. After vertical marker 434, the cylinders transition toa single injection of fuel beginning at 432.

Intake manifold pressure (MAP) during the engine start is indicated bytrace 436. When the engine is started, MAP is at atmospheric pressurebecause air enters the intake system at engine stop by passing throughthe throttle body and entering the intake manifold. As the engine beginsto rotate, air in the intake manifold is drawn into cylinders, therebyreducing the intake manifold pressure. The intake manifold pressurestabilizes after the engine reaches idle speed.

Vertical marker 434 indicates engine conditions when engine speed isabove a threshold speed and when manifold pressure is below a threshold.In this example, engine speed is greater than the threshold speed beforeMAP is below a threshold MAP. Thus, vertical marker 434 indicates athreshold MAP for this engine start. The threshold MAP may vary withoperating conditions. For example, threshold MAP may lower as ambientaltitude increases. Conversely, threshold MAP may increase as ambientaltitude decreases. As indicated by FIG. 4, fuel injection istransitioned from injecting twice per cylinder cycle before 434, toinjecting once per cylinder cycle after 436. Fuel is transitioned fromtwo injections per cylinder cycle, which may improve combustionstability, to one injection per cylinder cycle to reduce engineemissions. A single fuel injection during the intake stroke of acylinder may improve engine emissions because fuel mixing may beimproved by injecting once during an intake stroke.

Referring now to FIG. 5, a flow chart of an engine starting routine isshown. At 502, routine 500 determines engine operating conditions.Engine operating conditions may include but are not limited to enginecoolant temperature, fuel type or percent ethanol, fuel rail pressure,ambient temperature and pressure, transmission gear position,transmission oil temperature, engine throttle position, acceleratorpedal position, and brake pedal position. The various engine operatingconditions may be directly sensed or inferred from a combination ofsensors for example.

At 504, routine 500 judges whether or not there is an automatic enginerestart request. In one example, an engine start request may begenerated after a vehicle is automatically stopped in response tovehicle speed below a threshold speed and a depressed brake pedal. Theautomatic engine start request may be generated by the operatorreleasing the vehicle brake pedal or engaging/releasing the clutch pedalor when the accelerator pedal is depressed. In other examples, anautomatic start request may be generated by a hybrid controller forexample. An automatic start request may be generated for a vehicle withan automatic transmission or a manual transmission. If the engine iscoupled to an automatic transmission, the transmission may be in a drivegear or in park or neutral. If an automatic engine restart is requestedroutine 500 proceeds to 506. Otherwise, routine 500 proceeds to exit.

At 506, fuel may be injected into a cylinder that is on a compressionstroke before the engine rotates. However, if the engine is not in aposition desirable for starting, fuel injection may be delayed until theengine begins to rotate.

At engine stop, a cylinder counter is set to a value of zero. In oneexample, the cylinder counter may be incremented by the first instanceof fuel injection and as each cylinder passes though abottom-dead-center intake stroke position. In other embodiments, thecylinder counter may be incremented at top-dead-center intake stroke ofeach cylinder or at alternate engine positions. Thus, the cylindercounter is zero while the engine is stopped and increments as the enginebegins to rotate moving the respective engine cylinders through theindividual cylinder cycles.

Returning to 506, engine rotation begins and cylinder counting isinitiated by routine 500. The engine rotation may be accomplished by astarter or by a motor of a hybrid vehicle.

At 508, routine 500 judges whether or not IMEP limitations warrantentering skip fire mode. IMEP may be limited by injector minimum pulsewidth limitations, ambient environmental conditions, or by other sensoror actuator limitations. In one example, IMEP may be limited by thequantity of air inducted into an engine cylinder, and by the number ofengine cylinders. Further, the IMEP limitation may increase as theamount of air held by an engine cylinder increases. For example, engineshaving a greater number of cylinders and larger cylinder volumes mayhave higher IMEP limits as compared to engines having fewer cylindersand lower volume cylinders.

In one embodiment, IMEP limitations of an engine may be determined witha dynamometer and stored in memory of a manufactured vehicle. Forexample, an engine may be operated under various conditions to determineduring what conditions cylinder IMEP is greater than a value that allowsthe engine to operate at a desired idle speed. In one example, an enginemay operate at a higher IMEP when ambient air temperature is cold andwhen engine temperature is warm. The colder ambient air temperatures mayincrease cylinder air charge such that the warm engine will have ahigher torque output. In another example, IMEP may be higher than isdesired when a lean air-fuel mixture is desired and when a fuel injectoris operating at a minimum pulse width resulting in an engine torqueoutput that is higher than is desired.

When an automatic start request is made under engine operatingconditions that were previously determined to result in IMEP higher thandesired, routine 500 proceeds to 510. Otherwise, routine 500 proceeds to520.

At 510, routine 500 judges whether to inject fuel to a cylinder once ortwice each cylinder cycle. In one example, fuel may be injected once percylinder cycle during an engine start when a temperature or fuel railpressure of the engine is greater than a threshold. In this example,fuel may be injected twice per cylinder cycle during an engine startwhen a temperature or fuel rail pressure of the engine is less than athreshold. Injecting fuel twice during a cylinder cycle during an enginestart may improve combustion stability under some conditions. Whereas,injecting fuel once during a cylinder cycle during an engine start mayimprove engine emissions.

In another example, fuel may be injected once per cylinder cycle duringan engine start when an ambient air pressure is greater than athreshold. In this example, fuel may be injected twice per cylindercycle during an engine start when an ambient air pressure is less than athreshold.

At 512, routine 500 injects fuel to each cylinder once during the cycleof each of the engine cylinders and in the order of engine combustion.For example, fuel may be injected during the intake stroke of eachcylinder of a four cylinder engine in an order of 1-3-4-2. Further, at512, routine 500 decides which cylinder to prohibit fuel injection toduring a cycle of the cylinder in response to a value of the cylindercounter. As a result of prohibiting fuel injection during a cycle of acylinder, the engine may skip one or more combustion events. In oneexample, fuel may not be injected to a cylinder when the cylindercounter matches a number stored in memory of an engine controller.Therefore, one or more combustion events may skip combustion beforecombustion resumes. In another example, fuel may not be injected to acylinder at predetermined intervals, every third cylinder in the orderof engine combustion for example. Thus, as the engine rotates andcombusts air-fuel mixtures, the cylinder counter is incremented. Whenthe cylinder counter reaches predetermined numbers, the fuel injectionmay be temporarily stopped so that engine torque is reduced, therebycontrolling engine speed. As mentioned above, the cylinder counter maybe incremented at specific cylinder events (e.g., a piston attop-dead-center intake stroke), engine combustion events (e.g., when aspark is initiated), or other instances related to engine position.After 512 is complete, routine 500 proceeds to 516.

At 514, routine 500 injects fuel to each cylinder twice during the cycleof each of the engine cylinders in the order of engine combustion. Forexample, fuel may be injected twice during the compression stroke ofeach cylinder of a four cylinder engine in an order of 1-3-4-2. Further,at 514 routine 500 decides which cylinder to prohibit fuel injection toduring a cycle of the cylinder in response to a value of the cylindercounter. As a result of prohibiting fuel injection during a cycle of acylinder, the engine may skip one or more combustion events. In oneexample, fuel may not be injected to a cylinder when the cylindercounter matches a number stored in memory of an engine controller.Therefore, one or more combustion events may skip combustion beforecombustion resumes. In another example, fuel may not be injected to acylinder at predetermined intervals, every third cylinder in order ofengine combustion for example. Thus, as the engine rotates and combustsair-fuel mixtures, the cylinder counter is incremented. When thecylinder counter reaches predetermined numbers, the fuel injection maybe temporarily stopped so that engine torque is reduced, therebycontrolling engine speed. As mentioned above, the cylinder counter maybe incremented at specific cylinder events (e.g., a piston attop-dead-center intake stroke), engine combustion events (e.g., when aspark is initiated), or other instances related to engine position.After 514 is complete, routine 500 proceeds to 516.

Note that the error between the desired and actual engine speed may bedetermined on a cylinder event by cylinder event basis, same as theevent counter, and can also be used to determine when to skip a cylindercombustion event on to not skip a cylinder combustion event. Emissionsimplications may also be considered. For example, an engine may belimited to a predetermined number of skipped combustion events eachengine start. Further, a monitor of the number of skipped combustionevents or a ratio of skipped combustion events over firing events may beprovided to determine when to exit skip mode combustion.

At 516, routine 500 judges whether or not to exit skip fire mode. In oneembodiment, skip fire mode may be exited when the cylinder counterreaches a predetermined number since engine stop. For example, skip firemode may be exited after a predetermined number of cylinder events orcombustion events since engine stop. In another example, skip fire modemay be exited after a predetermined amount of time or after an operatorinput for additional engine torque. For example, routine 500 maytransition out of skip fire mode when an operator depresses anaccelerator pedal. If routine 500 judges to exit skip fire mode, routine500 proceeds to 518. Otherwise, routine 500 returns to 510.

At 518, routine 500 exits skip fire mode by continuously injecting fuelto cylinders and by providing spark to each cylinder in order of enginecombustion order. Further, engine speed may be controlled by adjustingspark or reducing cylinder air charge after exiting skip fire mode.After 518, routine 500 exits.

At 520, routine 500 judges whether to inject fuel to a cylinder once ortwice each cylinder cycle. In one example, fuel may be injected once percylinder cycle during an engine start when a temperature or fuel railpressure of the engine is greater than a threshold. In this example,fuel may be injected twice per cylinder cycle during an engine startwhen a temperature or fuel rail pressure of the engine is less than athreshold. Injecting fuel twice during a cylinder cycle during an enginestart may improve combustion stability under some conditions. Whereas,injecting fuel once during a cylinder cycle during an engine start mayimprove engine emissions.

In another example, fuel may be injected once per cylinder cycle duringan engine start when an ambient air pressure is greater than athreshold. In this example, fuel may be injected twice per cylindercycle during an engine start when an ambient air pressure is less than athreshold.

At 522, routine 500 injects fuel to each cylinder once during the cycleof each of the engine cylinders and in the order of engine combustion.In one embodiment, the engine may be started with a lean air-fuelmixture as is described by FIG. 3. In particular, the engine may bestarted by injecting fuel during the compression stroke to form astratified mixture near or around the spark plug just prior toinitiating a spark event. Further, the engine may be operated stratifiedlean for a predetermined number of cylinder events or combustion eventssince engine stop.

In one embodiment, engine cylinders are operated with lean air-fuelmixtures according to the order of combustion until the cylinder counterreaches a predetermined number. Further, the predetermined number may bevaried in response to engine operating conditions. For example, theengine may operate lean for twenty combustion events when enginetemperature is near 20° C. and for three events when engine temperatureis near 90° C. Thus, as the engine rotates and combusts air-fuelmixtures, the cylinder counter is incremented. After 522, routine 500proceeds to 526.

At 524, routine 500 injects fuel to each cylinder twice during the cycleof each of the engine cylinders in the order of engine combustion. Forexample, fuel may be injected twice during the compression stroke ofeach cylinder of a four cylinder engine in an order of 1-3-4-2. After524, routine 500 proceeds to 526.

At 526, routine 500 judges whether or not to exit start mode. In oneembodiment, start mode may be exited when the cylinder counter reaches apredetermined number since engine stop. For example, start mode may beexited after a predetermined number of cylinder events or combustionevents since engine stop. In another example, start mode may be exitedafter a predetermined amount of time or after an operator input foradditional engine torque or if the intake manifold pressure drops belowa specified threshold pressure. For example, routine 500 may transitionout of start mode when an operator depresses an accelerator pedal. Ifroutine 500 judges to exit start mode, routine 500 proceeds to exit.Otherwise, routine 500 returns to 520.

Referring now to FIG. 6, a flow chart of an alternate engine startingroutine is shown. At 602, routine 600 determines engine operatingconditions. Engine operating conditions may include but are not limitedto engine coolant temperature, fuel type or percent ethanol, fuel railpressure, ambient temperature and pressure, transmission gear position,transmission oil temperature, engine throttle position, acceleratorpedal position, and brake pedal position. The various engine operatingconditions may be directly sensed or inferred from a combination ofsensors for example.

At 604, method 600 judges whether or not there is an automatic enginerestart request. In one example, an engine start request may begenerated after a vehicle is automatically stopped in response tovehicle speed below a threshold speed and a depressed brake pedal. Theautomatic engine start request may be generated by the operatorreleasing the vehicle brake pedal or when the accelerator pedal isdepressed. In other examples, an automatic start request may begenerated by a hybrid controller for example. Further, in oneembodiment, the engine may be restarted when there is lack of operatorinput to an engine torque demand input.

An automatic start request may be generated for a vehicle with anautomatic or manual transmission. If the engine is coupled to anautomatic transmission, the transmission may be in a drive gear or inpark or neutral.

In one embodiment, fuel may be injected to one or more cylinders on acompression stroke in response to a request for an automatic start.Further, the injection may continue as the engine begins to rotate inresponse to the request to automatically start the engine. If anautomatic engine restart is requested routine 600 proceeds to 606.Otherwise, routine 600 proceeds to exit.

At engine stop, a cylinder counter is set to a value of zero. In oneexample, the cylinder counter may be incremented by the first instanceof fuel injection and as each cylinder passes though abottom-dead-center intake stroke position. In other embodiments, thecylinder counter may be incremented at top-dead-center intake stroke ofeach cylinder or at alternate engine positions. Thus, the cylindercounter is zero while the engine is stopped and increments as the enginebegins to rotate moving the respective engine cylinders through theindividual cylinder cycles.

Returning to 606, engine rotation begins and cylinder counting isinitiated by routine 600. The engine rotation may be accomplished by astarter or by a motor of a hybrid vehicle.

At 608, routine 600 injects fuel to each cylinder twice during the cycleof each of the engine cylinders in the order of engine combustion. Forexample, fuel may be injected twice during the compression stroke ofeach cylinder of a four cylinder engine in an order of 1-3-4-2. Further,fuel may be injected twice during the compression stroke of each enginecylinder for a predetermined number of cylinder events or combustionevents and then fuel may be injected twice, one injection during theintake stroke of a cylinder, the second injection during the compressionstroke of the cylinder. In this way, the engine may be started byinjecting twice during a compression stroke a predetermined number oftimes and then transitioned into injecting fuel twice during a cylindercycle for each engine cylinder, the first injection during the intakestroke and the second injection during the compression stroke. Inaddition, the amount of fuel may be varied depending on a temperature ofthe engine and on the MAP. It should be noted that a third injection offuel during a cycle of a cylinder during an engine start is alsopossible. Routine 600 proceeds to 610 after the fuel delivery method hasbeen selected and commanded such that fuel is injected twice percylinder cycle for the respective cylinders.

At 610, routine 600 judges whether or not engine speed is greater than athreshold amount while fuel is injected twice during a stroke of therespective cylinders. Routine 600 judges whether engine speed is greaterthan a threshold as a condition to establish that fuel injection can betransitioned from a fuel injection method that may improve combustionstability to a fuel injection method that may improve engine emissions.Further, the engine speed threshold may vary for depending on engineoperating conditions. For example, as a temperature or fuel railpressure of the engine increases the engine speed threshold maydecrease. If engine speed exceeds a predetermined threshold, thethreshold varying with engine operating conditions, routine 600 proceedsto 616. Otherwise, routine 600 proceeds to 612.

At 612, routine 600 judges whether or not engine MAP is less than athreshold amount while fuel is injected twice during a stroke of therespective cylinders. Routine 600 judges whether MAP is less than athreshold to determine if the engine may be misfiring. If MAP is notless than a threshold, routine 600 proceeds to 614 and fuel is enrichedto improve the possibility of stable combustion during engine start.Otherwise, routine 600 returns to 608.

At 614, routine 600 richens the air-fuel mixtures of engine cylinders toreduce the possibility of misfires. When the engine is not misfiring,engine speed increases and the intake manifold pressure is reduced.However, if an engine cylinder misfires, engine speed may be reducedsuch that less air may be drawn from the intake manifold resulting inhigher intake manifold pressure. The amount of fuel injected to enginecylinders is increased at 614 to reduce the possibility of enginemisfire.

At 616, routine 600 judges whether or not engine MAP is less than athreshold level. Engine MAP may be one indication of engine startingrobustness. If the intake manifold pressure is pumped down as expected,the engine may be at an operating condition that is stable enough totransition to a fuel injection method that may improve engine emissions.Therefore, routine 600 judges whether or not MAP is less than athreshold. If MAP is less than a threshold, routine 600 proceeds to 618.Otherwise, routine 600 returns to 608 where the engine continuescombusting air-fuel mixtures that include two separate injections offuel during the respective cylinder cycles as shown in FIG. 4, forexample.

At 618, routine 600 transitions to injecting fuel once to each of therespective cylinders during a cylinder cycle. Fuel injected once percylinder cycle may improve engine emissions during an engine start. FIG.4 shows a representative transition from injecting twice per cylindercycle into each cylinder to injecting once per cylinder cycle into eachcylinder. Specifically, after 434 of FIG. 4, fuel is injected once percylinder cycle for a cylinder that has not started an injection cycle(e.g., an injection cycle may be an injection period to inject all fuelfor a single combustion event during a single cylinder cycle).

At 618, routine 600 transitions fuel injection according to the enginecombustion order. For example, as shown in FIG. 4, the last cylinder tobegin receiving fuel before MAP is less than a threshold level at 434 iscylinder number 4 at 428. Cylinder number 4 receives the balance of thecylinder mixture at 430, and then, fuel injection is transitioned tosingle fuel injection per cylinder cycle in the order of combustion.Since cylinder number 2 is next in the order of engine combustion,cylinder number 2 is the first cylinder since engine stop to receivefuel once per cylinder cycle. Cylinders 1, 3, and 4 follow cylindernumber 2 transitioning to one injection event per cylinder cycle.Routine 600 exits once all cylinders have transitioned to one fuelinjection to each cylinder during a cylinder cycle.

The routine of FIG. 600 may also limit the entry into a torque controlmode until a predetermined number of cylinder events or combustionevents have occurred after an engine stop. In one example, routine 600may proceed from 618 to a state where fuel injection and operating modeare restricted from entering an engine torque control mode until aspecific predetermined number of combustion events are detected or untilan operator torque request is made. This restriction may improve thepossibility that the engine responds as desired when the engine entersan engine torque control mode.

It should be noted that in the examples of FIGS. 2-4, the engine may becoupled to an automatic transmission, but these methods of enginestarting may be appropriate for an engine coupled to a manualtransmission. Further, the present examples may be appropriate forstarting and engine coupled to an automatic transmission while thetransmission is in a drive gear, but the present methods may also beused to start an engine that is in neutral or park. Further still, theengine starting position of each example is shown merely forillustration purposes. The methods illustrated may be applied atdifferent engine starting positions and to engines having additional orfewer cylinders.

As will be appreciated by one of ordinary skill in the art, routinesdescribed in FIG. 4 may represent one or more of any number ofprocessing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various steps orfunctions illustrated may be performed in the sequence illustrated, inparallel, or in some cases omitted. Likewise, the order of processing isnot necessarily required to achieve the objects, features, andadvantages described herein, but is provided for ease of illustrationand description. Although not explicitly illustrated, one of ordinaryskill in the art will recognize that one or more of the illustratedsteps or functions may be repeatedly performed depending on theparticular strategy being used.

This concludes the description. The reading of it by those skilled inthe art would bring to mind many alterations and modifications withoutdeparting from the spirit and the scope of the description. For example,I3, I4, I5, V6, V8, V10, and V12 engines operating in natural gas,gasoline, diesel, or alternative fuel configurations could use thepresent description to advantage.

The invention claimed is:
 1. A method for starting an engine coupled toan automatic transmission, comprising: stopping the engine;automatically initiating an engine restart with the transmission in gearand initiating combustion in a first engine cylinder; and skippingcombustion, according to an order of combustion of the engine, in atleast one cylinder of the engine during the engine restart afterinitiating combustion in the first cylinder.
 2. The method of claim 1wherein engine speed is controlled in response to a torquecharacteristic of a torque converter coupled to the engine.
 3. Themethod of claim 1 wherein the skipped combustion is in response to alevel of IMEP of cylinders combusting an air-fuel mixture since theengine stop.
 4. The method of claim 1 wherein skipping combustion in theat least one cylinder is in response to a number of engine cylinderssince combustion in the first cylinder.
 5. The method of claim 1 whereinsaid skipping of combustion in the at least one cylinder is in responseto a predetermined condition.
 6. The method of claim 5 wherein thepredetermined condition is a temperature of the engine and whereinskipping combustion in the at least one cylinder of the engine is inresponse to a number of combustion events since combustion in the firstcylinder.
 7. The method of claim 1 wherein the skipping combustion inthe at least one cylinder is limited to a number of cylinder intakeevents after combustion in the first cylinder.
 8. A method for startingan engine coupled to an automatic transmission, comprising: stopping theengine; during a first condition, automatically initiating an enginerestart with the transmission in gear and initiating combustion in afirst cylinder of the engine, a speed of the engine controlled byadjusting one of at least engine spark timing, intake manifold pressure,or engine fuel amount; and during a second condition, different than thefirst condition, automatically initiating an engine restart with thetransmission in gear and initiating combustion in a first cylinder ofthe engine, and skipping combustion, according to an order of combustionof the engine, in at least one cylinder of the engine during the enginerestart after initiating combustion in the first cylinder.
 9. The methodof claim 8 wherein combustion is initiated in the first cylinder byinjecting fuel into a cylinder having closed intake valves before enginerotation after the engine stop.
 10. The method of claim 8 wherein theskipped combustion is in response to a level of IMEP of cylinderscombusting an air-fuel mixture since the engine stop.
 11. The method ofclaim 8 wherein skipping combustion in the at least one cylinder is inresponse to a number of engine cylinders since combustion in the firstcylinder.
 12. The method of claim 8 wherein said skipping of combustionin the at least one cylinder is in response to a predeterminedcondition.
 13. The method of claim 12 wherein the predeterminedcondition is a temperature or a difference between a desired enginespeed and an actual engine speed, or fuel rail pressure, or intakemanifold pressure, or ambient pressure of the engine and whereinskipping combustion in the at least one cylinder of the engine is inresponse to a number of combustion events since combustion in the firstcylinder.
 14. The method of claim 8 wherein during the first and secondconditions, engine speed is controlled in response to a torquecharacteristic of a torque converter coupled to the engine.
 15. A methodfor starting an engine, comprising: stopping the engine while anautomatic transmission coupled to the engine is in a drive gear;automatically initiating an engine restart and initiating combustion ina first engine cylinder with multiple fuel injections while theautomatic transmission is in gear; and skipping combustion, according toan order of combustion of the engine, in an engine cylinder during theengine restart after initiating combustion in the first cylinder. 16.The method of claim 15 wherein the engine stopping is automaticallyinitiated by an engine controller.
 17. The method of claim 15 whereinthe drive gear is a first gear.
 18. The method of claim 15 whereincombustion is initiated in the first cylinder by injecting fuel into acylinder having closed intake valves before engine rotation after theengine stop.
 19. The method of claim 18 wherein a starter is engaged tostart the engine.
 20. The method of claim 15 wherein the skippingcombustion in the at least one cylinder is limited to a number ofcylinder intake events after combustion in the first cylinder.